.box {
    width: 100wh;
    height: 100vh;
    text-align: center;
    background: linear-gradient(#FFCCCC, #CCFF99);
    position: relative;
}

.box .form {
    position: absolute;
    width: 400px;
    height: 300px;
    top: 300px;
    left: 700px;
}